Aluminum Catches Up to Carbon Composites

The race to
carbon composites in aircraft structures is anything but a sure bet, says a top
executive at Alcoa, America's leading aluminum producer.

"Industry
perceptions have changed very dramatically over the last three to five years
from favoring composites to being fundamentally a dead heat," says Bill
Christopher, executive vice president at the Pittsburgh-based company. "To
paraphrase Mark Twain, we believe that the rumors of the demise of the aluminum
structure are greatly exaggerated."

He based his
statements on research conducted recently by Alcoa and pointed to three recent
aircraft designs as proof.

Designed for
the 100- to 149-seat market category, The Bombardier C Series uses large
amounts of composites like the Boeing Dreamliner and the Airbus A350. But its
airframe makes intensive use of new lighter weight aluminum lithium alloys
developed by Alcoa. The first flight of the Canadian aircraft is expected next
year.

Another
regional jet, the Chinese-built ARJ21 also has an
aluminum-intensive airframe.

The initial
design of the Japanese Mitsubishi
Regional Jet called for composite wings but engineers shifted to aluminum
"for a lot of very good reasons," says Christopher.

Officials at
the OEM said that use of an aluminum wing box will allow for a shorter
lead-time to make structural changes. The wings, for example, can be optimized
to match the attributes of each member of the MRJ airplane family. First flight
is expected next year.

"The next
battle ground will be a single-aisle aircraft," says Christopher. "For shorter
routes, weight reductions (from carbon composites) don't translate into as much
savings." Another issue is that regional, single-aisle aircraft take off and
land more frequently than large planes such as the Dreamliner and A350. They
take more punishment.

Christopher
says that aluminum has more damage tolerance than carbon composites. "Also, you
can see damage in aluminum much easier and repair on aluminum is much easier,"
he adds.

One big advantage
of carbon composite structures is a requirement for less frequent Inspection interval
cycles. Alcoa has developed new aluminum alloys that have improved corrosion
resistant and are 5 to 10 percent lighter than the alloys that were available
when design decisions were made from the Dreamliner. And more gains are coming
with aluminum-lithium alloys under development, advanced structural concepts
and other new technologies being developed by Alcoa researchers, according to
Christopher.

One new
structural concept is called selective reinforcement. Aluminum-lithium sheet
reinforced with glass fibers is placed in specific areas that need to be strengthened.

Christopher
says that Alcoa's next-generation structural technology will be less risky and
30 percent less-expensive to manufacture, operate and repair than current
generation carbon composite structures. In his opinion, aluminum will also catch up to
carbon composites on passenger comfort areas such as large windows, higher
humidity, and higher cabin pressure.

However, improvements,
of course, are also coming in carbon composite systems.

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